A custom post type named shooting has a field (with ACF) with plenty of images. Those are listed on the single.php of the CPT and one of those images is displayed in full-ish size. I currently have URLs like shooting/foo-bar/?image=1 where 1 is the index (not ID) of the image that should be displayed large.
Is there a way to get prettier URLs like shooting/foo-bar/1 with WordPress? Or would I have to write a custom rewrite-rule for that?
You would need a custom rewrite rule for that:
http://codex.wordpress.org/Rewrite_API/add_rewrite_rule
I think you might have a hard time though, with it being within an area of the site (single post type) already using a custom rewrite rule!